Managing the risk of cracking in heat treating
Tool and die components are subjected to intense stresses before their final desired hardness is achieved. While the science behind hardening is complicated, customers’ demands are simple: Improve my material without destroying it.
There are many reasons cracks can occur during treatment, but tool steels are at enhanced risk due to their high hardenability. That’s why tool steel design is so critical to reducing that risk. Understanding how design can influence cracking risk and monitoring parts for design characteristics that could promote cracks are the most effective ways to prevent failure.
It’s all about cross section
A tool or die’s shape can make or (literally) break it during heat treating. Hardening these components first requires temperatures high enough to cause a transformation in the metal’s crystal structure. During the cooling that follows, the part surface cools and hardens more quickly than the core. As the core hardens, it expands, putting pressure on the part surface. Drastic changes in a part’s cross section present cracking risks because the stress focuses around points where these sudden changes occur.
Consider these common design characteristics:
Thin webs of metal between much larger masses pose a significant cracking risk due to abrupt changes in the part’s cross section. During heating, the thinner portion will reach the desired treatment temperature much faster than the larger parts. During cooling, it will cool and transform much more quickly. These stress differentials all but assure the part will crack. The best way to avoid this is to design parts without thin webs.
Another example of a thin web is the area of a stamping that lies between two holes machined closely together. To avoid cracks, heat treaters can pack the holes with furnace insulation, diminishing the cracking risk by making parts with holes in them behave as though they had none.
Knife edges are thin, sharp slivers of metal often found in holes in a die when machining doesn’t punch completely through it. Avoid this by inspecting dies thoroughly before heat treating and either rejecting or reworking them when knife edges are detected.
Lack of radii refers to cross section transitions that are not smooth or parts that contain corners that are not sufficiently rounded. Parts designed with a lack of radii are at greater risk of cracking. Designing parts that gradually taper from one size to another and that feature well-rounded corners reduces cracking risk as heat treatment stresses are applied more uniformly across them. Consult a metallurgist for more information about designing with proper radii.
Under special conditions, using dummy blocks in notches, gaps or corners of dies acts in a similar way to furnace insulation, diminishing the cracking risk by making them temporarily more uniform during heat treatment.
Welding on dies and tool components prior to heat treatment can set the stage for cracking. The intense heat of welding puts significant local stress on an area of a part surrounded by parts not under stress. Also, if weld puddles aren’t smoothed over via machining, the rough shape increases stress concentration, increasing the chance for a crack to form.
A common misconception is that only the areas of tool and die components that display the above problems will be affected by cracking. However, while these areas may represent only tiny portions of a part, the cracks that begin are likely to rip through an entire piece.
Monitoring and communication
While understanding how design characteristics of tool and die components influence cracking risk, part owners, engineers and heat treaters must actively manage those risks from design through production and final treatment.
Heat treating specialists at Paulo maintain open lines of communication with part owners and engineers, educating them on the heat treatment process and developing heat treatment plans that achieve desired qualities at minimum risk.
Sometimes, designs are just too risky. While metallurgists will notice this and alert the proper parties, part owners can save time and expense by learning how design influences cracking risk before their materials ever reach a furnace.
At Paulo, we maintain a wide variety of certifications that prove we provide quality work on every job. Managing the risk of cracking during heat treatment is just one of many areas of expertise we offer as an outsourced heat treater. Download the guide below to learn more.